I've been "tolerating" this exact same problem with my 1975 Batavus HS-50. The nut that holds the clutch assembly onto the crackshaft is tight but the clutch assembly turns without the engine turning over. It seems the clutch kas a key on it that is worn off. Might yours be having a similar issue? I could get a beautiful spark when I took the plug out and tested it but the engine couldn't turn over once there was compression. I had to manually craft a locking key that would keep the clutch in place. So far it's working.